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Full text of "Handbook Of Chemical Engineering - I"

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424                              CHEMICAL ENGINEERING
Thus when both instruments read correctly for an external resistance of 2 ohms if the external resistance is increased by an ohm, the indicator of low resistance is in error by 7.7 per cent or about 77°C. at 1,000°C., while the instrument of high resistance still reads practically correct. Corrections such as the above emphasize the importance of using a galvanometer having a resistance of 300 ohms or more. In actual operation the line resistance may change by several ohms on account of bad contacts and the deterioration of the thermocouple. The error due to variable line resistance is eliminated in the Harrison-Foote compensated indicator described later, even though the resistance of the indicator is low.
Use of Portable Test Set.—On account of the errors which may be introduced in the reading of an uncompensated, simple galvanometer when the resistance of the line or couple varies, it is important to have some means for measuring this resistance occasionally. This is, of course, especially necessary when the resistance of the galvanometer is low. Every plant maintaining a large thermocouple installation with simple galvanometrie indicators should have a portable wheatstone bridge or "test" set for this purpose. A simple and inexpensive instrument is made by the Leeds & Northrup Co.
In measuring the resistance of the line care must be taken, if two indicators or an indicator and recorder are operated in parallel on the same couple, to see that both instruments are disconnected from the circuit during the measurement of the resistance. Otherwise the instrument left connected acts as a shunt upon the line. If the resistance of the line and couple is found to be much higher than that for which the indicator was designed, short circuit the line at the cold junction and determine whether the fault is in the couple or the line. If in the former it usually means that the couple is broken or requires replacement. By making occasional observations of this kind serious faults may be detected long before they would be suspected from the resulting low values in the indicated temperatures.
Galvanometer with Variable Series Resistance.—As shown above, galvanometers, especially those of low resistance, are usually calibrated to read correctly for a definite line resistance. Suppose an indicator is desired for operation on a line of which the resistance changes from practically zero to 10 ohms. The instrument is calibrated to read correctly for a line resistance of 10 ohms. Located in the case of the galvanometer and in series with the line is a variable resistance from zero to 10 ohms. As the resistance of the line increases the variable resistance is decreased by manual adjustment so that the sum of the line resistance and the variable series resistance always equals 10 ohms. The dial of the variable resistance is graduated to read the amount of resistance cut out of the circuit. Hence, it should be set at the resistance of the line and couple.
This value may be determined by means of a test set as discussed above. Thus the method, which has been employed by Siemens & Halske and by Weston, is of great value for precision work with a galvanometric indicator. The principal objection to it, which applies to all galvanometric indicators so far described when used for accurate measurements is the necessity of measuring the resistance of the line and couple, this latter measurement requiring the use of a test set or similar device. This objection is eliminated in the following instrument, and other desirable features have been added.
Harrison-Foote Compensated Indicator.—This instrument manufactured by the Brown Instrument Co. is illustrated in simple form by Fig. 5. The circuit CDGF is an ordinary millivoltmeter in which G represents the moving coil.